The invention relates to a drive gear arrangement of an electronically controlled wristwatch whose operating mechanism includes a stepping motor having a rotor shaft provided with a drive structure which is coupled by way of an intermediate drive gear and center gear and reducing gear structures to the watch's minute hand gear.
The manufacture of such an operating mechanism requires a plurality of work operations which must be executed with great precision and which are therefore quite tedious and expensive. This is especially true for today's wristwatches which the customer wishes to be quite small and especially very slim. It is also to be taken into consideration that the various components of the operating mechanism must be manufactured with great precision and the whole gearing mechanism must be assembled with the same precision such that the gears and pinions of the operating mechanism are in precise engagement. The various shafts must be spaced from one another correctly and they need to be perfectly parallel and cooperating pairs of gears should be in engagement with one another over the full desired width of the gear structure. In the past with relatively large watches it was possible to provide relatively wide pinions to insure that the associated gear discs were in full gear width engagement but with today's slim watches also the pinions are to be made relatively narrow so that axial positioning of the various gears has become a matter of great importance.
Such considerations also apply with regard to the intermediate gear structure and the second gear disc and also with regard to the engagement of the second gear pinion and the disc of the reducing gear structure disposed between the second hand gear and the minute hand gear.
The invention concerns especially these two pairs of gears, that is, the engagement of the intermediate gear and the second or center gear structure and also of the second gear structure with the reducing gear because these gearing structures are determining factors as far as the thickness of a watch is concerned since the second gear needs to be located in the center of the gear arrangement if the watch is to be provided with a second hand. Then there is an overlapping of intermediate gear structure, second gear structure, reducing gear structure, minute gear structure and hour gear structure and the intermediate gear structure, the second gear structure and the reducing gear structure need to have relatively wide pinions in order to insure proper engagement of the appropriate gear discs without requiring overly accurate positioning and guiding of the gears.
The required transmission ratio provided between these gears depends on the rotational steps per second of the stepping motor, that is, on the number of pulses per second supplied to the stepping motor by the electronic control circuitry.
If the presence of a second hand is desired, then the transmission ratio between the second gear and the minute gear is 60:1. However, if the watch has no second hand, the transmission ratio between the second hand gear, that is then the center gear, and the minute gear may be selected on the basis of other considerations. Then it is of course also unimportant by which step length the center gear advances, it does not have to be by the second angle of 6.degree. per second.